Ok, this is the challenge. My boat came with a batterycharger that after all these years still works fine.

When I run the frig at the dock, the power demand is 8amps or 80% of the max my charger supplies. As a result, after a while the charger gets hot, and trips out. Aside from that the charger still works fine. So replacing it with a new one is a waste of money.

With that as a starting point, how would you wire shore power to a 12VDC AB unit that draws about 8+ amps?

One starts with a 120VAC > 12VDC 20AMP rectifier. If you connect the rectifier and the normal DC feed to the cooling unit, when the rectifier is feeding the unit, it will back-feed to the batteries, and overcharge them. Likewise when on DC, the DC would back-feed the rectifier. I believe the result is the rectifier acting as an inverter and back-feeding the AC circuit or at the least it would not be good for the rectifier's circuitry.

My thinking was to add diodes to both the rectifier and the DC cables to prevent back-feeds.

I would like to keep the rectifier feed isolated from the boat's DC system. By switching or protecting both the pos and neg, I get 100% isolation. Am I overthinking that part?

The other option is a PURE two pole a/b switch. Unfortunately, I cannot find one.

Using a battery switch allows for both A&B to be selected, this would result in fried batteries. Not an acceptable outcome. They also only switch the pos.

Any thoughts??

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"Whenever...it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off- then, I account it high time to get to sea..." Ismael -a link to my delivery website is in my profileó

...When I run the frig at the dock, the power demand is 8amps or 80% of the max my charger supplies. As a result, after a while the charger gets hot, and trips out. Aside from that the charger still works fine. So replacing it with a new one is a waste of money...

Your logic is seriously flawed.

The conclusion is that adding the refrigerator made the old charger obsolete, as it is no longer up to the task of keeping your batteries charged.

Replace the charger with a smart charger of adequate capacity.

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__________________1st rule of yachting: When a collision is unavoidable, aim for something cheap.
"whatever spare parts you bring, you'll never need"--goboatingnow
"Id rather drown than have computers take over my life."--d design

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"Whenever...it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off- then, I account it high time to get to sea..." Ismael -a link to my delivery website is in my profileó

Careful with those cheap 12V rectifiers (your term), actually they are power supplies. They can get quite warm even at lower output currents, output voltage may not be that well regulated and they are electrically noisy (static on the radio). Add an inline fuse to the output just to be safe.

A switch is a better idea than the diodes. Use a DPDT (double pole double throw) toggle switch, a 20A rated one with screw terminals should be < $10 from an electronics supply house such as Digikey or Newark.

Your 10A battery charger is a bit on the light side and you should consider a replacement.

Will your refrigerator run off a rectifier? Exactly what brand /model rectifier are you looking at? If I'm not mistaken, a 12V refrigerator will require a steady and well-regulated DC supply, and a "rectifier" won't deliver this. Yes, you can use diodes to isolate the rectifier output from the battery/charger output, but you will have a moderately significant voltage drop from the diodes.

Backfeeding the rectifier from the battery will not generate AC at the rectifier input, but I suppose something could be damaged.

Does your refrigerator somehow have an AC power option?

It sounds like you need expert advice, and upgrading your charger is still probably going to be the best solution.

The charger provides 10amps to each battery. It Is actually a 20, but it will not send 20 to one battery. So far it meets my needs.

A well regulated rectifier will send 12vdc. But the point about heat is well made.

Sounds like I need to spend some quality time with AB customer support. I can't be the first person with this question.

Cheers

Sent from my iPhone- please forgive autocorrect errors.

__________________
"Whenever...it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off- then, I account it high time to get to sea..." Ismael -a link to my delivery website is in my profileó

__________________
"Whenever...it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off- then, I account it high time to get to sea..." Ismael -a link to my delivery website is in my profileó

__________________1st rule of yachting: When a collision is unavoidable, aim for something cheap.
"whatever spare parts you bring, you'll never need"--goboatingnow
"Id rather drown than have computers take over my life."--d design

My boat came with a smart charger. Which didn't work. Took it home, plugged it into the mains and, voila, it worked. Took it back to the boat, tested it, green light! Fitted it and three days later it failed. I took it out and handed it to a marineengineer. He said it would cost more to repair than buying a new one

I had an automotive smart battery charger in the back of the car and used it as my power supply on the boat for quite a while. Cost me about GBP40. It would supply plenty of power for the fridge and charge my batteries when hooked up to the shore. A bit Heath Robinson but.....

The electronics inside the charger stopped it from frying the batteries. I also used the output from my solar panels so it worked for me.